LC 73矩阵置零Set Matrix Zeroes　中等

若某元素为 0，则把它所在行和列全部置 0，要求原地、常数空间。

思路 首行首列做标记数组：内层按标记置零，再用两个变量处理首行首列自身。空间 O(1)。

class Solution:
    def setZeroes(self, matrix: List[List[int]]) -> None:
        m, n = len(matrix), len(matrix[0])
        first_row = any(matrix[0][j] == 0 for j in range(n))
        first_col = any(matrix[i][0] == 0 for i in range(m))
        for i in range(1, m):  # 首行首列记标记
            for j in range(1, n):
                if matrix[i][j] == 0:
                    matrix[i][0] = matrix[0][j] = 0
        for i in range(1, m):  # 按标记置零内部
            for j in range(1, n):
                if matrix[i][0] == 0 or matrix[0][j] == 0:
                    matrix[i][j] = 0
        if first_row:
            for j in range(n):
                matrix[0][j] = 0
        if first_col:
            for i in range(m):
                matrix[i][0] = 0